The long term objective is to determine the molecular mechanism by which vasopressin and norepinephrine stimulate the contraction of vascular smooth muscle. Vasopressin has long been known to be released into the blood during hemorrhage and to stimulate vascular contractions that maintain blood pressure above dangerously low levels. Vasopressin has also been used clinically to treat patients at risk from internal hemorrhages. Yet the mechanism of vasopressin-stimulated vascular contraction has been quite unclear. Recently much evidence has been published that vasopressin increases cytosolic Ca2+ levels in liver by means of receptor-mediated breakdown of phosphatidyl inositol 4,5 bis phosphate, followed by inositol 1,4,5 triphosphate stimulated Ca+2 release from endoplasmic reticulum. Some evidence has been published that vasopressin might exert similar effects on vascular smooth muscle, thereby increasing Ca+2 levels within the smooth muscle and stimulating vascular contraction. The effects of vasopressin on phosphatidyl inositol 4,5 biphosphate levels in rat or rabbit aortal smooth muscle will be determined in this proposal. We will determine directly whether or not vasopressin stimulates the breakdown of inositol-containing phospholipid in this tissue. The importance of these effects will be determined with the assistance of neomycin. Since neomycin can block inositol lipid metabolism, it should block vasopressin-stimulated smooth muscle contraction if inositol lipid metabolism is critical for these effects. The role of GTP-binding proteins and phorbol ester-binding protein kinase C in this system will also be explored. The proteins directly responsible for vasopressin-stimulated inositol lipid metabolism in vascular smooth muscle will be eventually isolated and the system reconstituted with the purified components.